System phenol

If the third substance dissolves in only one of the liquids, it is found that their mutual solubilities are decreased and the C.S.T. is generally raised. For example, a concentration of 0 15 mol of potassium chloride per litre of water raises the C.S.T. of the water - phenol system by about 12° a similar concentration of naphthalene in the phenol produces a rise of about 30°. 

In the presence of carbon black the phenols and phenol/DLTP combinations are much less effective whilst some phenolic sulphides (e.g. Santonox) show positive synergism with carbon black. Flowever, in general terms the phenol systems tend to be reduced to about the same levels as to those to which the phenol sulphide systems are raised. Some typical figures are given in Table 10.8. 

It would not be unreasonable to suggest that the dipole moment of the phenolic system has a negative skew toward the aromatic hydroxyl and that the electronic repulsion of these groups in an ortho-ortho situation greatly reduces the likelihood... 

Standard-grade PSAs are usually made from styrene-butadiene rubber (SBR), natural rubber, or blends thereof in solution. In addition to rubbers, polyacrylates, polymethylacrylates, polyfvinyl ethers), polychloroprene, and polyisobutenes are often components of the system ([198], pp. 25-39). These are often modified with phenolic resins, or resins based on rosin esters, coumarones, or hydrocarbons. Phenolic resins improve temperature resistance, solvent resistance, and cohesive strength of PSA ([196], pp. 276-278). Antioxidants and tackifiers are also essential components. Sometimes the tackifier will be a lower molecular weight component of the high polymer system. The phenolic resins may be standard resoles, alkyl phenolics, or terpene-phenolic systems ([198], pp. 25-39 and 80-81). Pressure-sensitive dispersions are normally comprised of special acrylic ester copolymers with resin modifiers. The high polymer base used determines adhesive and cohesive properties of the PSA. 

Table 3. The log 1 /Kd(X) values for cyclodextrin-substituted phenol systems at pH 7.0 and 25 °C and the parameters of substituents...

Matsui et al.82) have analyzed the same data of log l/Kd(X) for cyclodextrin-phenol systems from a somewhat different standpoint. They computed the minimal van der Waals interaction energies (Emin) for the systems by using the same method as described in a previous section (Table 4). The calculated Emin values were applied, in place of such steric parameters as Ibrnch and B1( to the correlation analysis. The correlations obtained are given in Eqs. 24 to 27. 

Table 4. Van der Waals energy for cyclodextrin-phenol systems (—Em(n kJ/mol)...

Only the hydrophobic and steric terms were involved in these equations. There are a few differences between these equations and the corresponding equations for cyclo-dextrin-substituted phenol systems. However, it is not necessarily required that the mechanism for complexation between cyclodextrin and phenyl acetates be the same as that for cyclodextrin-phenol systems. The kinetically determined Kj values are concerned only with productive forms of inclusion complexes. The productive forms may be similar in structure to the tetrahedral intermediates of the reactions. To attain such geometry, the penetration of substituents of phenyl acetates into the cyclodextrin cavity must be shallow, compared with the cases of the corresponding phenol systems, so that the hydrogen bonding between the substituents of phenyl acetates and the C-6 hydroxyl groups of cyclodextrin may be impossible. 

Ester-cured alkaline phenolic system. The resin is an alkaline phenolic resin (essentially the same as the self-hardening resins of this type). Sand is mixed with the resin and blown or manually packed into a core box. A vaporized ester, methyl formate, is passed through the sand, hardening the binder. The total resin and peroxide addition is 1.5%. Compression strengths of 5000 kPa (700 psi) are possible. 

Early mechanistic studies have indicated that the oxypalladation step in the Wacker process proceeds through an <37z/z-pathway,399 although recent deuterium-labeling experiments have shown the viability of a yy/z-mechanism involving insertion of a metal-coordinated oxygen into the alkene.400,401 For example, with excess chloride ion present, the Wacker-type cyclization of a deuterated phenol system occurred in a primarily //-pathway, whereas the oxypalladation step favored a yy/z-mode in the absence of excess chloride ion (Scheme 16). Thus, either mechanism may be operative under a given set of experimental conditions. 

The last step of the reaction is the keto-enol tautomerization from T 4-cyclohexadienone intermediates (15) to aromatic products (16). Such a step is accompanied with a considerable gain in energy about 80 kJ mol 1 for vinylcarbenes [29], (where a phenol system is formed by the tautomerization step), and about 175 kJ mol 1 for phenylcarbenes [25] (where a naphtol system is produced). The energy barrier for such step should be lower than 40 kJ mol 1 according to previous calculations on similar systems [42],... 

In the ionization of the p-nitroanilinium ion, the free base is stabilized by delocalization of electrons involving the canonical structure 13. An analogous structure is not possible for the p-nitroanilinium ion. In the ionization of p-nitrophenol, analogous delocalization is possible in both phenol and phenate species, but is more marked in the ion. Thus, in both the aniline and the phenol system P-NO2 is effectively more electron-attracting than in the ionization of benzoic acid, where the reaction centre is incapable of a —R effect, and indeed shows a small +R effect (14). 

Figure 13.6. Equilibrium relationship for the aniline-water-phenol system...

Ring protonation has been looked for in some other phenolic systems also. Thus a C-protonated cation is observed at low temperature when 2,4,6-trihydroxybenzoic acid is dissolved in 70% perchloric acid and pure fluorosulphuric acid (Birchall et al., 1964). A... 

Many natural products are produced by the coupling of two or more phenolic systems, in a process readily rationalized by means of radical reactions. The reactions can be brought about by oxidase enzymes,... 

Flavonoids are natural phenolic systems containing pyrylium and pyrone rings, and provide the most prominent examples. We have met some of these systems under antioxidants (see Box 9.2). Coumarins contain a 2-pyrone system. Note that all of these compounds are fused to a benzene ring and are strictly benzopyran or benzopyrylium systems. 

The mechanism of imine formation is standard, as seen in the other examples. The cyclization reaction is then like the Mannich reaction, attack of an enol on to the iminium cation. This time though, the nucleophile is provided by the resonance effect from the phenol system. 

Proton NMR spectra of coal derivatives generally give rise to either broad peaks or complicated multiplets which can be easily divided into band envelopes. For example, the H1 spectrum of a coal-hydrogenation asphaltene (4) consists of three peaks, two of which overlap. A broad peak at lowest field is caused by protons in aromatic and phenolic systems, whereas two higher field peaks are caused by protons bonded to carbons situated o to aromatic rings and those bonded to other nonaromatic carbons, respectively. The ratios of these spectral areas are the same as the ratios of the hydrogens in each of these three hydrogen classes. This accounts for one of the most important characteristics of proton NMR spectra—namely, no calibration data are necessary. 

The simple phenol system has been discussed here at some length, because material balance is obtained and mechanistic details are fairly well understood. However, according to the data in Tables 3.5 and 3.6, there is a very noticeable gap in the material balance in the case of the hydroxylated benzoic acids, although some aspects such as the acid-catalyzed water elimination, in salicylate also more pronounced in the case of the para-OH-adduct radical, are very similar (Mark and von Sonntag, unpubl.). Interestingly, addition of Fe(III) to oxidize the intermediates also did not improve the material balance (Tables 3.5 and 3.6). From this, it follows that the underlying chemistry of the salicylate and the other hydroxybenzoate systems are at present not yet adequately understood, and the... 

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